The present invention relates to an image sensing method and apparatus and, more particularly, to an image sensing method and apparatus capable of sensing a document and a drawing such as an illustration in high resolution.
Conventionally, many kinds of image sensing devices, such as a video camera and an electronic still camera, capable of sensing natural pictures in high quality have been developed and supplied in the market. Recently, image sensing devices have been developed to serve as information collecting devices, since information processing devices, such as a computer, have been personalized. Accordingly, there is a demand for the image sensing devices having ability to sense characters, numerals, figures, and so on, in high quality.
In order to sense a document and a drawing in high quality, it is necessary to properly control the light exposure. However, a problem in variation of exposure level arises by variation in character density in a document.
If the exposure level varies as described above, contrast between sensed character parts and background parts in the document become unclear. Therefore, there is a problem in that characters in picked-up images cannot be identified in many cases.
When a natural picture is sensed, it will be sensed in almost good quality by adjusting an integrated value of image signal of the picture inside of an area of light measurement (after .gamma. process) to about a level of 50% to 60% of a dynamic range of an image signal. In contrast, if a document with black letters on white paper is sensed at the same level, an image signal level of the white parts is too low to identify characters or to be used in an OCR device.
Further, if the color and brightness of the paper of the document differ from those of white paper, and if the same image sensing condition for sensing the black characters in the white paper is used to sense the document, difference between levels of character parts and of background parts may not be large enough. In this case, problems occur in that the character parts and the background parts cannot be separated from each other and that characters maybe sensed in improper forms in the successive processes.